For the transmission of high voltage (such as in the range of several hundreds of kV) direct current (DC), specific types of cables have been developed which take the particular requirements associated with DC transmission into account. These cables are known as high voltage direct current cables (HVDC cables), and their structure comprises essentially the following elements from the inside to the outside in a radial direction:                a conducting core or conductor, including a multitude of individual conductors such as copper or aluminium; this conductor is the high-voltage (HV) element;        a conductor screen (“semiconductor layer”, generally called a “semicon”) which can be made of an electrically conducting polymer layer, e.g. a polymer layer filled with carbon particles; the polymer layer is generally called an “inner semicon”;        an insulation layer which can be made of cross-linked polymers such as polyethylene;        an insulation screen which can be made of an electrically conducting polymer layer, e.g. a polymer layer filled with carbon particles (“semiconductor layer”); this element is held on ground potential and is generally called an “outer semicon”;        shielding and protecting layers which essentially do not contribute to the actual insulation of the high-voltage (HV) element.        
In such a conductor, the inner semicon, the insulation layer and the outer semicon can be provided as one single polymer layer which is made in an extrusion process. The inner semicon layer and the outer semicon layer are made by providing the radially innermost and the radially outermost regions of this polymer layer with a corresponding carbon black filler.
In alternating current (AC) electricity transport, for which differences in the dielectric constant of different insulating materials are crucial in the distribution of the electric field around high voltage parts, the dielectric constant generally does not vary more than by a factor of 1-3 for commonly used materials. In contrast to AC electricity transport, for high voltage direct current transport, the resistivities of the insulating materials are of greater significance than the dielectric constants of the insulating materials, which can vary by factors of 100 or more. This phenomenon creates an impetus for a more careful analysis of the electric field distribution, such as in a situation where cables are connected in a way that the insulation is not simply terminated in a cable termination element but has to be continued in a cable joint. The variation in the resistivity is particularly problematic in view of the fact that resistivities show a high temperature dependence, and as a result, the matching of resistivity of insulation materials may not be possible for all temperatures.
Accordingly, specific prefabricated cable joints have been developed for such HVDC cables which ensure that the distortions of the electric field distribution in the insulating material are small in order to avoid field enhancements and concomitant electric breakdown.
It is known to use tape-based cable joining technology for flexible connections between HVDC cables. Flexible joints for HVDC light cables can be made by lapping and successively vulcanizing semi-conductive and insulating tapes around the cable. Then, the lead and polyethylene jacket sheaths, and the armouring are mounted. The joint-cable interface can have a conical shape.
Among other things, there is the consideration that the electric field distribution in the interface region between the cable insulation material and the joint insulation material is made as homogenous as possible. According to conventional techniques, the homogeneity of the interface region can be obtained by matching the electrical properties of the joint insulation and the cable insulation as close as possible. In particular, the resistivity ratio, which determines the field distribution in the cable joint region at DC voltages, is made close to one. However, this requires a high quality and extremely well controlled process.